The invention relates to a method and circuit for receiving low voltage differential signals (LVDS).
Differential amplifiers are widely used to pick out small data signal voltages from larger noise voltages. This is accomplished by arranging the larger undesired signal to be the common-mode input voltage of a differential amplifier, and arranging the smaller data signal to be the differential input voltage. In this way, a differential amplifier can output an amplified version of the differential input voltage, that is the desired data signal, while cancelling any noise which has been introduced onto the differential signal during transmission. Low voltage-differential signals are used in many applications, for example telecommunications, where the data signal is susceptible to noise during transmission.
According to IEEE standard 1569.3-1996 for low voltage differential signals, differential transmission equipment should transmit signals with differential voltages of xc2x1250 mV to xc2x1400 mV and common mode voltages of 1.2V. Due to the fact that a receiver may have a different ground potential from an associated transmitter, the reception equipment has to be capable of receiving these signals with an extended common mode voltage range of between 0V to 2.4V to allow for possible deviation in the common mode voltage.
However, conventional differential circuits have difficulty in detecting differential signals over such a wide common mode voltage range, particularly when the supply voltage of the receiver is limited to 3V minimum, since the 2.4V range approaches the 3V supply voltage.
In addition, conventional differential circuits can only derive the differential signal from an input differential signal with a common mode voltage of about 1.2V or above. Conventional high speed differential circuits have difficulty deriving differential signals having a common mode voltage below this value, since the transistors of the detection circuitry fail to operate properly, (ie. they do not have enough voltage to turn them ON), and the current sources saturate.
Furthermore, complementary amplifiers used in conventional differential circuits normally use integrated PNP transistors in situations where the input common mode voltage is low. However, such transistors cannot be used in high frequency applications, because their cut-off frequency ft is usually less than the bandwidth of the application. Consequently, applications in the telecommunications field, when implemented in bipolar or bicmos technology, tend to use NPN transistors because their bandwidths are typically of the order of 100 times higher than bandwidths of PNP transistors.
Thus, for the reasons mentioned above, conventional differential circuits are not suitable for receiving low voltage differential signals, particularly in high frequency applications.
GB 2297211 discloses a differential signal receiver circuit which increases the common mode voltage range using a second differential amplifier fed through a level shifter, with the output current of first and second differential amplifiers summed to provide complementary signals. This circuit uses p-channel devices in the signal path (or pnp if implemented in bipolar technology), thus having the bandwidth disadvantages mentioned earlier.
U.S. Pat. No. 5,889,419 discloses another circuit having an improved common mode range. A first comparator is activated when the input voltages are above a first level, while a second comparator is activated when the input voltages are below a second level. However, the activation of the first and second comparators is provided by voltage level-sensing circuitry having hysteresis, which produces discontinuities in the output signal at the region where the circuit switches over from one comparator to another.
The aim of the present invention is to provide a low voltage differential signal receiver which operates over a wide common mode voltage range, and which has a linear transfer characteristic, even over the amplifier transition region.
According to a first aspect of the invention, there is provided a circuit for receiving a low voltage differential signal having a common mode voltage range, the circuit comprising:
a first amplifier circuit which operates in a first voltage sub-range of the common mode voltage range:
a second amplifier circuit which operates in a second voltage sub-range of the common mode voltage range, and,
a control circuit for controlling the operation of the first and second amplifier circuits so as to provide an output signal over the common mode voltage range of the input signal, wherein the control circuit sums the output of the first and second amplifiers in an overlapping region of the first and second voltage sub-ranges.
According to a second aspect of the invention, there is provided a method of receiving a low voltage differential signal having a common mode voltage range, the method comprising the steps of:
providing a first amplifier circuit for operating in a first voltage sub-range of the common mode voltage range:
providing a second amplifier circuit for operating in a second voltage sub-range of the common mode voltage range, and,
controlling the operation of the first and second amplifier circuits so as to provide an output signal over the common mode voltage range of the input signal, wherein the output signals from the first and second amplifiers are summed in an overlapping region of the first and second voltage sub-ranges.